Re: Dual sine wave generator with variable frequency and 90 degree phase difference


"Spehro Pefhany" <speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:ll80c4d3j6pvcpe4ql5077m7faj62sve1m@xxxxxxxxxx
On Wed, 03 Sep 2008 21:22:18 -0700, JosephKK <quiettechblue@xxxxxxxxx>
wrote:

On Wed, 3 Sep 2008 12:30:05 -0700 (PDT), miso@xxxxxxxxx wrote:

On Sep 3, 11:46 am, Leon <leon...@xxxxxxxxxxxxxx> wrote:
On 3 Sep, 14:42, Steve <st...@xxxxxxxxxx> wrote:

I'm looking for a waveform generator that outputs two sine waves of
the same frequency with 90 degree phase difference (sine and cosine).
I need a variable frequency between 0.05 Hz and 10 Hz. Is there an
analog design that uses a single potentiometer or perhaps is voltage
controlled ? Low distortion is not a requirement.

Steve

I've used a software DDS running on an MCU with a DAC for that sort of
thing. It's much easer to design and build than any analogue circuit.

Leon

This all depends on the frequency resolution, i.e. VCO step size. A
coordic would be a better approach.

I'm always amazed at how often the coordic is not suggested as a
solution. I suspect it isn't taught in college courses anymore. I
haven't found a good write up on the net regarding the algorithm
either. It's decades old technology.

To find it use CORDIC (COrdinate Rotation by Digital Integrating
^^^^
ITYM COordinate. ;-)

Computation ). For sine / cosine DDA is simpler. At 8-bits lookup
tables are easier still. It will leave you time for really fancy pwm
methods to do error spectrum shifting / noise shaping like better CD
and semipro (and above) audio does.

Besides, a uC DDS solution allows variable phase relationships that
most other solutions have trouble with.

You only need to store one quadrant of sin(x) for both sin and cos
eg. 0 <= x <= pi/2, so the lookup table can be compact.



You can use up pi/4 because cos(2x) = 2cos(x)^2 - 1

In fact you can get arbitrarily small lookup tables and use the identities
to compute the rest. So one trades speed for size.







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